1. Field of the Invention
The present invention relates to a multi-layer, high-temperature corrosion protection coat for metallic surfaces.
2. Description of the Prior Art
Corrosion protection coatings for high operating temperatures are generally used in machine construction. The main field of application of such high-temperature protection coatings is found in the area of thermal fluid flow engines, particularly on components subject to high stress, such as gas turbine blades. These coatings serve the purpose of extending the life of the protected high-temperature materials.
The protective coatings known to the prior art are, in general, based on the protective effect of the oxides of chromium, aluminum and silicon, as well as alloying elements (yttrium), either individually or in combination (see for example, U.S. Pat. No. 3,676,085; U.S. Pat. No. 2,754,903; U.S. Pat. No. 3,542,530; German Pat. No. 2,520,192). Prior art coatings are also based on silicate layers based on Ni/Cr/Si/B alloys (see for example, Villat, M., Felix, P., "High-Temperature Corrosion Protection Coating for Gas Turbines", Technische Rundschau Sulzer 3, 1976, Pages 97 to 104).
The customary corrosion protection coatings for high-temperature applications are mostly specifically designed for resistance against certain corrosive agents. However, in the cases of corrosion by a multiplicity of agents, the anti-corrosive behavior of prior art coats is usually unsatisfactory. Thus, the protective coatings made from Cr, Al and Si have generally favorable characteristics in oxidizing atmospheres but fail in the presence of relatively high amounts of sulfur and fuel gases.
Because of their poor resistance to sulfidization, such prior art coatings require the use of relatively pure fuels, a fact which restricts their field of application. Additionally, such protective coatings are often deficient in chemical-physical compatibility with the base material to be protected, whereby the coatings tend to crack and peel. On the other hand, coatings on the basis of Ni/Cr/Si/B are generally quite compatible with the base material but do not have optimum corrosion behavior.
A need therefore continues to exist for a high-temperature corrosion protection coating with staggered protective effect for high operating temperatures which has increased sulfidization resistance with good oxidation resistance at high temperatures. Such a protective coating should have good physical-chemical compatibility with the base material which it covers and should be suitable for the production of solid solutions.